In the first terrorist murder on the British mainland since the 7/7 suicide bombings of 2005, the men attempted to behead the soldier, hacking at him like a “piece of meat” in front of dozens of witnesses, before both were shot by police who took around 20 minutes to arrive.

After the killing, one of the men, believed to be a British-born Muslim convert, spoke calmly into a witness’s video phone.

One of the attackers speaks into a witnes's camera phone following the attack (ITV)

Speaking with a London accent, holding a knife and a meat cleaver and with his hands dripping with blood, he said: “We swear by almighty Allah we will never stop fighting you until you leave us alone. Your people will never be safe. The only reason we have done this is because Muslims are dying by British soldiers every day.

“We must fight them as they fight us. An eye for an eye and a tooth for a tooth. I apologise that women had to witness this today but in our lands our women have to see the same. You people will never be safe. Remove your government, they don’t care about you. Do you think David Cameron is going to get caught in the street when we start busting our guns? Do you think your politicians are going to die?

“No, it’s going to be the average guy like you, and your children. So get rid of them. Tell them to bring our troops back so we, so you can all live in peace.”

Witnesses said that the men used a car to run over the soldier just yards from the Royal Artillery Barracks in Woolwich, south-east London, before setting about him with knives and a meat cleaver as if they were “trying to remove organs”. One unconfirmed report suggested that he had been beheaded.

Passers-by said they thought at first that the attackers were trying to help the man, who was wearing a Help for Heroes T-shirt, and only realised they were killing him when they got closer.

As they attacked the soldier, one of the men shouted “Allahu akbar”, or God is Great, according to the BBC, while another witness said they appeared to pray next to the body as if the solder was a “sacrifice”.

Their victim, thought to be aged around 20, had reportedly been on duty at an Army recruitment office in central London and was on his way back to the barracks when he was murdered at 2.20pm.

It emerged that passers-by went to the soldier’s aid. One of the killers ordered that only women could tend to the body, not men.

There were also questions over why it took around 20 minutes for armed police to arrive on the scene, during which time the killers calmly walked up and down the road, carrying their bloodied knives and a pistol, while members of the public confronted them.

When police did arrive, both gunmen tried to rush at the police and were shot, reportedly by a female officer.

On Wednesday night they were under armed guard in separate hospitals. Their British accents suggested that they were “home-grown” terrorists and security sources said they did not believe anyone else was involved in the incident.

David Cameron described the attack, which had chilling echoes of a plot to behead a soldier foiled in Birmingham in 2007, as “absolutely sickening”, but said that Britain will “never buckle” in the face of terrorism. This morning he will chair a meeting of the Government’s Cobra emergency briefing committee to be updated on developments.

Speaking in Paris, where he had been meeting François Hollande, the French president, he said: “We have suffered these attacks before. We have always beat them back. We have done that through a combination of vigilance, of security, of security information, good policing.

“But above all, the way we have beaten them back is showing an absolutely indomitable British spirit that we will not be cowed, we will never buckle under these attacks. The terrorists will never win because they can never beat the values we hold dear, the belief in freedom, in democracy, in free speech, in our British values, Western values. They are never going to defeat those. That is how we will stand up to these people, whoever they are, however many there are of them, and that is how we will win.”

He added that “every aspect” of security would be reviewed. After a Cobra meeting last night, chaired by Theresa May, the Home Secretary, security was tightened at all London barracks.

Boris Johnson, the Mayor of London who attended the meeting, said: “I know that Londoners have been through terrorism before and this city has huge resilience.

“What we also have is the best, the most professional security services and the best police in the world to protect us and they are now going to get to the bottom of exactly what’s happened.”

The Queen announced that she would go ahead with a planned visit to the King’s Troop Royal Horse Artillery at Woolwich Barracks next week.

The murder appeared to have been planned to ensure maximum publicity, with the killers urging witnesses to take their picture “as if they wanted to be on TV”. One witness, identified only as James, said he and his partner watched in horror as they realised what they were seeing. He shouted at the men to stop, only for one of them to pull out a gun and threaten to shoot him.

After driving his car further up the road, he stopped and called the police, telling them to bring armed units.

He said: “These two guys are chopping this guy to pieces, literally hacking at something like it’s a bit of meat. These two guys were crazed, they were just animals. They then dragged the poor guy from the pavement and dumped his body in the middle of the road.

“They were standing there with the knives in their hand, waving the gun about. There were police at the end of the road but there were no police in the vicinity of the attackers. I think they were proud of what they were doing.”

Footage of the incident obtained by ITV News showed one woman crouched over the body of the dead soldier in an attempt to shield it from further attack. Three other women stood between the body and the killers.

Joe Tallant, a van loader who lives near the scene, said: “My friend and her mum were walking up the hill and the mum came straight to the victim.

“She asked the black guys, 'Can I help him?’ And one of them said he was already dead but she could have a go. Then one of them said, 'No man is coming near this body, only women.’

“She was so brave, she didn’t care what happened to her; she knelt down by his side and comforted him. She held his hand and put her other hand on his chest.”

Mulgrave Primary School, just yards from the scene of the killing, went into “lockdown”. However, parents said some of the children had witnessed the attack.

The Muslim Council of Britain condemned the killing as a “truly barbaric” act with “no basis in Islam”.

A spokesman said: “We call on all our communities, Muslim and non-Muslim, to come together in solidarity to ensure the forces of hatred do not prevail.”